schlez said

I made sure that PDF is set to "Preview in Firefox in Options. When setting it to "Open in Firefox (default)", it just opens tabs endlessly.

Yes, you definitely want "Preview in Firefox" and not Firefox chasing its tail sending the PDF to itself as an external application -- which is what causes all those unwanted windows/tabs.

Before, when I double clicked a PDF in File Explorer or opened a PDF in a link, the PDF would open in the in-built Firefox Reader. After the update, it just keeps on redownloading/copying the file.

I don't understand what's happening. What do you mean by redownloading/copying?

jscher2000 said

I don't understand what's happening. What do you mean by redownloading/copying?

I just mean that (for example) if I open a PDF from File Explorer, it just makes a copy of it in my "Downloads" folder and shows me an empty tab in the browser. And when I open a PDF from a link, it just downloads it and shows me an empty tab.

Is the local file:// or web address in the bar as expected?

If so, it sounds as though Firefox opens the PDF but for some reason does not render out its contents.

Could you check for error messages in the Browser Console. You can open that using Ctrl+Shift+j. After a few moments, lots of text will appear. Click the garbage can at the upper left of the console to remove those messages.

Then leaving that window aside, switch back to your document and reload the PDF (you can use Ctrl+r or click the reload button). Then check back in the console for any errors related to the PDF. Could you copy/paste anything you discover?

Actually I got it working just now.

I downloaded another PDF reader and set that as the default application. Then I noticed for some reason, if I right-clicked the PDF -> "Open With" -> Firefox, the PDF opened fine. Now I set Firefox back as the default PDF reader and it's working again.

Anyways thank you for all the help jscher2000!

Problem: when you select and right click on a pdf file on the desktop or in explorer Firefox is not offered as an option to open it. To get Firefox to be offered as an option do the following; right click on a pdf file/open with/choose another app, Click it. In the box that comes up scroll down to more apps (with a little down arrow next to it). Click it, Again scroll down to Look for another app on this PC, Click it. An Open with... box will open already in Program Files, Navigate to Mozilla Firefox, double click, then click firefox.exe and then open at the bottom, the PDF will now open in Firefox. Now right click on another PDF file/open with, Firefox will now be offered. To get Firefox to be the DEFAULT program to open PDF files when they are double clicked, again right click on a pdf file/open with/choose another app, Click it. In the box that comes up click Always use this app to open .pdf files, then click on Firefox. Done. Simples, worked for me in Win 10 Pro.